The present invention generally relates to an optical network device, and more particularly, to an optical network device or unit of a passive optical network capable of magnetically adjusting the power of an upstream optical signal to be outputted to an optical line termination of the passive optical network.
Optical devices or units are devices that process optical signals in connection with an optical network. An example of employing optical units is for facilitating the communication between a user terminal and an optical network. FIG. 1 is a schematic diagram illustrating a prior art passive optical network (PON). Referring to FIG. 1, a passive optical network 10 includes an optical line termination OLT, an optical distribution network ODN, and a plurality of optical network units ONU1-ONUn. The optical line termination OLT, which can be at a central office (CO) connected to external networks, can provide various network services, such as Internet, digital television, high definition television (HDTV) or Voice over Internet Protocol (VOIP). The optical distribution network ODN may include optical fibers and an optical splitter. The optical splitter of the optical distribution network ODN can distribute downstream data from the optical line termination OLT to one or more of the optical network units, or collect upstream data to be transmitted from one or more of the optical network units to the optical line termination OLT. The optical network units ONU1-ONUn are normally placed at customer ends, also known as customer remises equipments (CPEs). Depending on the locations of the optical network units ONU1-ONUn, the passive optical network 10 can be categorized into fiber to the home (FTTH), fiber to the building (FTTB), fiber to the curb (FTTC), or fiber to the premises (FTTP) networks.
For communication from the optical line termination OLT to one or several of the optical network units ONU1-ONUn, data are simultaneously transmitted downstream through a fiber. For communication from one or several of the optical network units ONU1-ONUn to the optical line termination OLT, data are transmitted upstream using the same wavelength and obtained from the same fiber by the optical splitter of the optical distribution network ODN. Therefore, a time division multiplexing (TDM) scheme is employed so that each of the optical network units ONU1-ONUn can upload data to the optical line termination OLT during its corresponding time slot. Since the distances between the optical line termination OLT and the optical network units ONU1-ONUn may vary, the upstream and downstream signals may encounter different degrees of optical signal power loss. Although the downstream signals from the optical line termination OLT to the optical network units ONU1-ONUn may have different power levels upon arrival, an optical receiver provided in each of the optical network units ONU1-ONUn generally is able to process individual downstream signals. However, it may be difficult for an optical receiver of the optical line termination OLT to process incoming upstream signals from the optical network units ONU1-ONUn having different power levels. To obtain accurate data from the corresponding optical network unit, the optical line termination OLT sometimes needs to be able to determine the peak power of each upstream signal.
U.S. Patent Publication No. 20040247246, entitled “OPTICAL POWER EQUALIZER IN A PASSIVE OPTICAL NETWORK”, discloses an optical power equalizer capable of equalizing the power levels of the upstream signals outputted by a plurality of optical network units. The optical power equalizer may include an amplifier circuit able to provide gain adjustment in real time. However, the use of amplifiers circuit may raise cost concerns in some applications.
In U.S. Patent Publication No. 20050244160, entitled “OPTICAL TRANSCEIVER FOR COMPENSATING FOR LOSS DUE TO TRANSMISSION DISTANCE IN PASSIVE OPTICAL NETWORK”, the distances between an optical line termination and a plurality of optical network units are respectively calculated based on the peak values of downstream signals received by the corresponding optical network units. Each optical network unit can then adjust the output power of an upstream signal by adjusting the bias current of a laser module.
In IEEE Photonics Technology Letters, Vol. 14, No. 11, pp. 1560-1562, Nov. 2002, in a paper entitled “VARIABLE OPTICAL ATTENUATOR FOR LARGE-SCALE INTEGRATION” by Garner et al., a variable optical attenuator with a waveguide bend design is disclosed. The reflectivity of the waveguide can be varied by changing the temperature using an electrode heater, thereby varying the amount of signal attenuation.
In IEEE Journal of Lightwave Technology, Vol. 23, No. 5, pp. 1918-1922, May 2005, in a paper entitled “VARIABLE OPTICAL ATTENUATOR BASED ON ION-EXCHANGE TECHNOLOGY IN GLASS” by Ruschin et al, a variable optical attenuator based on the Mach-Zehnder (MZR) interferometer principle is disclosed. The variable optical attenuator includes two MZR optical paths having distinct lengths. The reflectivity of the MZR optical paths can be varied by changing the temperature using an electrode heater, thereby varying the signal interferences between the two MZR optical paths and the overall signal attenuation.
Depending on the applications and the devices or circuits used, certain applications in the past to adjust the power level of optical signals may raise cost concerns in some applications. In some applications or system designs, it may be desirable to have an optical network unit that is able to provide power level control without significant cost.